This invention relates generally to the field of synthetic polymeric coating compositions for polymeric and metal substrates; and, more particularly, to hydrophilic hydrogel coating compositions which are slippery and which exhibit tenacious adherence to the substrate to which they are applied, and to medical devices bearing such hydrogel coatings thereon. Still more particularly, this invention relates to hydrophilic polyurethane hydrogel compositions. The hydrogel coating compositions and coated substrate materials are biocompatible and suitable for use on medical devices which come in contact with various body fluids.
In catheters and many other kinds of medical devices, it is often desirable to coat various plastic, rubber or metal parts thereof with products made from hydrophilic or certain other polymers that are slippery and which produce low coefficients of friction during use. However, one of the problems associated with the utility of such coatings is their inability to remain intact and abrasion-resistant during clinical use in body fluids such as blood. Catheters used in angioplasty, gastroenterology and other medical specialties, are commonly made of polymeric materials which most often are relatively hydrophobic and not inherently slippery or biocompatible. A surface modification is required in order to reduce the friction between the catheter and other devices with which they work, such as vascular sheaths, and also to reduce the friction between the vasculature or other anatomical passageways and the catheter itself. Almost all currently used catheters have some form of surface modification or coating applied to them. The ability of the coating to reduce frictional resistance, its durability, as well as its biocompatibility are the most important functional aspects of an effective surface.
Heretofore, catheters and other medical devices containing synthetic or natural polymers have often been coated with non-permanent compositions such as silicones and other slip agents, fluorocarbons, or hydrogels which, however, were usually not cohesively attached to the substrate surfaces. While such coatings can impart a low coefficient of friction to the surface of a medical device, they typically lack permanence with respect to frictional wear. Fluorocarbons, moreover, may peel or flake from the substrate, or when applied to a soft polymeric substrate material, may cause an increase in the stiffness of the material. In the case of marginally polar substrates used for the fabrication of catheters and other medical devices such as contact lenses, condoms, gastroenteric feed tubes, endotracheal tubes, and the like, a variety of polyurethane based compositions have been suggested as adhesive tie coats. For such uses the coating must exhibit wear permanence, low coefficient of friction in contact with body fluids, as well extremely low toxicity and good biocompatibility. Whereas a number of polyurethane "tie coats" can improve adhesion to plastics and rubbers, they are oftentimes not compatible enough with respect to the polymer surface of the substrates to assure permanence of bonding for the intended medical application. In medical devices this can be a critical requirement for many clinical situations. Particular fields of medical specialties where such factors are important are enumerated below.
In Percutaneous Transluminal Coronary Angioplasty (PTCA) and Percutaneous Transluminal Angioplasty (PTA), the functional characteristics of balloon catheters include trackability through vasculature, crossability and recrossability of stenotic lesions, and retractability through the guiding catheter and the vascular sheath. These are dynamic functions that are fundamental to a successful and efficient interventional angioplasty procedure. They contribute to reduced trauma to the vasculature. In particular, recrossing of stenotic lesions is crucial to a successful outcome. High pressure angioplasty balloons, typically those made of polyethylene terephthalate (PET), can have problems with recrossability. This is because the relatively stiff PET material forms "wings" upon deflation after the first dilation. The winged profile of the deflated balloon can prevent recrossing of the stenotic lesion for a second dilatation. A durable slippery coating can aid in achieving recrossing of the lesion. Guiding catheters are better able to traverse tortuosity in the femoral artery and descending aorta with the help of a good slippery coating.
Stent catheters for use in vascular disease benefit from the characteristics imparted by a good slippery coating. Stent catheter delivery systems used in gastroenterology for opening of biliary passageways also benefit from a slippery coating with regard to traversing passageways leading to the site.
In coronary radiography, diagnostic catheters are used to deliver radiopaque fluid to the coronary arteries for visualization by x-ray fluoroscopy. These catheters benefit in the same way that guide catheters do from a good slippery coating, by aiding in traversing tortuosity in the femoral artery and the descending aorta.
U.S. Pat. No. 4,118,354 discloses the formation of polyurethane hydrogels which are reaction products of a polyisocyanate, having at least two isocyanate groups, and a polyether, produced from a plurality of alkylene oxides, 50 to 90% of which is ethylene oxide, added at random to a polyalcohol having at least two terminal hydroxyl groups, by the dispersal of the prepolymer reaction product into an aqueous liquid phase. Neither the formation of slippery hydrogel barrier coats upon plastic or metal substrates nor the affixation thereof to such substrates by means of covalent chemical bonds to assure durability of said coating upon exertion of dynamic forces thereon are described.
U.S. Pat. No. 4,373,009 describes a method for coating various polymeric substrates with polyurethane prepolymers containing free isocyanate groups and subjecting the thus coated substrates with a second coating of water-soluble copolymers of unsaturated monomers containing at least some isocyanate-reactive monomers as part of their backbone. It is postulated that the isocyanate treatment of the substrate results in firmly anchored tie coats even for polymers containing no isocyanate-reactive groups. No convincing evidence of covalent bonding of the urethane tie coat to the substrate is presented, nor is there any indication that the procedure is suitable for the use in critical medical devices where biocompatibility is a significant issue.
U.S. Pat. Nos. 4,459,317 and 4,487,808 discloses a process for treating a polymer substrate with a first coating of an isocyanate solution containing at least two unreacted isocyanate groups per molecule, and, optionally, a polymer; followed by a second coating of a high molecular weight polyethylene oxide, such that after curing of the isocyanate, the two coatings form a hydrophilic polyethylene oxide-polyurea interpolymer having a low coefficient of friction. Methods for applying a base coat of low molecular weight aromatic or aliphatic polyisocyanates dissolved in suitable organic solvents, followed by evaporating the solvent and then applying a second coat of a high molecular weight polyethyleneoxide polymer dissolved in an organic solvent are also disclosed. The second solution, which may also contain amine catalysts, is then evaporated and the two coatings are heated at elevated temperature in the presence of air which must contain enough moisture to react with the isocyanate of the first coating. The described processes are relatively time-consuming. The isocyanate coating is applied by spraying or dipping the substrate, and no evidence is presented that the isocyanate coating undergoes any reaction with the substrate surface to make it better adhering to the substrate surface. Medical devices made from a polymer substrate to which the coating has been applied, for use in body cavities, including especially the urethra, are also disclosed. Use of the coatings and coated medical devices in a blood medium, however, is not specifically disclosed, and it is believed that in the absence of bonding of the isocyanate coating to the substrate itself, the coatings and coated medical devices ultimately do not demonstrate the desired degree of permanence, especially in a blood environment.
U.S. Pat. No. 4,642,267 discloses a hydrophilic polymer blend which contains a thermoplastic polyurethane having no reactive isocyanate groups and a hydrophilic poly (N-vinyl lactam). The blend is said to be slippery in aqueous environments and is used as a low-friction coating for various substrates. Its use and performance in blood is not disclosed.
Published PCT Patent Application WO 89/09246 describes the use of shaped structures having polymer or metal substrate surfaces coated with crosslinked hydrophilic polymers, such as polyvinylpyrrolidone. The coated structures are said to be durable and exhibit a low coefficient of friction when wet. The use of polyethylene terephthalate (PET) substrates, which are often used in balloons for angioplasty catheters, is described. Crosslinking between the substrate and the coating is achieved by subjecting a hydrophilic polymer deposited on the substrate to thermally activated free radical initiators, UV light activated free radical initiation, or E-beam radiation. The adherence of the crosslinked hydrophilic polymer to the substrate surface is believed to be due to physical forces rather than to chemical bonding. A disadvantage of the process is that neither the thermally activated free radical initiators nor the UV initiators are biocompatible or suitable for medical uses. Furthermore, E-beam radiation applied to certain materials such as fluorocarbon polymers, which are often employed in medical devices, can be detrimental to these materials.
U.S. Pat. No. 4,990,357 describes coating compositions containing combinations of chain-extended hydrophilic thermoplastic polyetherurethane polymers with a variety of hydrophilic high molecular weight non-urethane polymers, such as polyvinylpyrrolidone. The coatings are made lubricious by contact with an aqueous liquid. The coatings adhere to a variety of polymeric substrates, including polyvinylchloride (PVC) and polyurethane (PU). A disadvantage of the coating compositions is that neither the thermoplastic polyurethane polymer, nor the hydrophilic non-urethane polymer can react with one another. Hence, it is not expected that these coatings give acceptable adhesion to most of the plastic substrates used in angioplasty devices.
U.S. Pat. No. 4,906,237 discloses the use of an osmolality-increasing compound such as glucose, sorbitol, sodium chloride, sodium citrate and sodium benzoate to improve the slipperiness and wetability of a surface coating for a polymeric substrate material which has first been coated with a non-reactive hydrophilic polymer. The coatings and coated substrates are said to be useful for situations where they come into contact with mucous membranes.
U.S. Pat. No. 5,026,607 describes the formation of a slippery coating of a urethane and a silicone or siloxane emulsion. A crosslinking agent, such as a polyfunctional aziridine, may be added to crosslink carboxyl functional groups in the coating with carboxyl functional groups on the substrate surface. The use of primers in the case of a PET substrate surface is also disclosed to effect better adhesion of the coating to the substrate. Alternative treatment methods to the use of primers, for example, the introduction of substrate surface functionality by means of plasma treatment or corona discharge to obtain hydroxyl, carboxyl, or amino functionality are also mentioned.
U.S. Pat. Nos. 5,077,352 and 5,179,174 describe the formation of lubricious coatings applied to a variety of substrates by means of forming crosslinked polyurethanes in the presence of polyethylene oxide polymers at high temperatures. No surface treatment of the substrate surfaces is described and the selection of the isocyanate compounds includes, in particular, reactive aromatic diisocyanates of the type not believed to be biocompatible. It is doubtful whether these methods can be recommended for use with intravenous catheter devices in view of the known carcinogenic nature of the amines which can result from the decomposition of such polyurethane polymers. Moreover, the high temperature polymerization procedures suggested can result in unacceptable physical changes of several of the polymeric materials utilized in angioplasty catheters.
Similar drawbacks pertain to the methods and compositions described in U.S. Pat. No. 5,160,790 describing the use of the same type of polyurethane polymers with various PVP polymers as the hydrophilic polymer species.
U.S. Pat. No. 5,132,108 discloses the use of plasma treatment of certain polymeric substrate surfaces, to introduce carboxyl and/or hydroxyl reactive groups thereon, utilizing an oxygen and water-containing plasma gas, followed by treating the resulting polymeric surface with a spacer component having amine groups. The treating step is conducted in the presence of a coupling agent, whereby covalent linkages are formed between the spacer component amine groups and the reactive sites of a modified hydrophilic polymeric substrate surface. Finally, an antithrombogenic, fibrinolytic or thrombolytic agent, such as heparin or other polysaccharides is contacted with the spacer component-treated modified polymeric surface. This method utilizes the introduction of relatively slow reacting carboxyl and/or hydroxyl groups onto the substrate surface, and encompasses too many processing steps for cost-effective production of medical devices. Although the resulting coated surfaces are biocompatible, they are not slippery and do not have low coefficients of friction.
U.S. Pat. No. 5,112,736 describes a method of introducing amino functionality on a variety of polymeric substrate surfaces, including polymers of polypropylene (PP), polyethylene (PE), polyvinylchloride (PVC), and polyvinylidenefluoride (PVDF), by plasma-treatment thereof in the presence of radiofrequency plasma discharge by means of ammonia, organic amine-containing gases, or mixtures of such plasma gases. The method is used for very hydrophobic hydrocarbon polymer articles such as PP membranes. It does not appear to give good results with PE polymers. PP films which contain amino groups on their surfaces are used for DNA sequencing on the membranes. No reference with respect to their use for attachment of hydrophilic PU polymers to highly hydrophobic substrates is made, nor does the reference disclose reliable methods to affix amino surface groups to PE surfaces which would be expected to work in the products and processes contemplated by the present invention.
The drastic influence of the chemical and physical composition of body fluids upon the permanence of low friction coatings when exposed to dynamic forces in such liquids has heretofore not been recognized. Whereas many slip additives, such as relatively low molecular weight silicones and a variety of hydrophilic polymers, exhibit slipperiness and relatively good permanence in the presence of water or saline solutions, they quickly lose their efficacy by exposure to dynamic forces in the presence of blood, a much more complex fluid composition.
Accordingly, there remains a need in the art of medical devices for an improved slippery coating material that demonstrates wear permanence, combined with the characteristics of biocompatibility, low toxicity and low coefficient of friction in contact with body fluids, especially blood.